Fast path for reading single doc with ordinals (#102902)

This optimization is added for enrich lookups, which are likely to match 
a single document. The change decreases the latency of the enrich
operation in the nyc_taxis benchmark from 100ms to 70ms. When combined
with #102901, it further reduces the latency to below 40ms, better than
the previous performance before the regression.

Relates #102625

docs/changelog/102902.yaml

@@ -0,0 +1,5 @@
+pr: 102902
+summary: Fast path for reading single doc with ordinals
+area: ES|QL
+type: enhancement
+issues: []

server/src/main/java/org/elasticsearch/index/mapper/BlockDocValuesReader.java

@@ -555,8 +555,20 @@ private static class SingletonOrdinals extends BlockDocValuesReader {
             this.ordinals = ordinals;
         }
 
+        private BlockLoader.Block readSingleDoc(BlockFactory factory, int docId) throws IOException {
+            if (ordinals.advanceExact(docId)) {
+                BytesRef v = ordinals.lookupOrd(ordinals.ordValue());
+                return factory.constantBytes(v);
+            } else {
+                return factory.constantNulls();
+            }
+        }
+
         @Override
         public BlockLoader.Block read(BlockFactory factory, Docs docs) throws IOException {
+            if (docs.count() == 1) {
+                return readSingleDoc(factory, docs.get(0));
+            }
             try (BlockLoader.SingletonOrdinalsBuilder builder = factory.singletonOrdinalsBuilder(ordinals, docs.count())) {
                 for (int i = 0; i < docs.count(); i++) {
                     int doc = docs.get(i);